3773
Inorg. Chem. 1988, 27, 3773-3781
Contribution from the Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
Acidity and Dimerization of Three Water-Soluble Iron( 111) Porphyrin Cations: (meso-a,P,a,P-Tetrakis(o - (N-methylnicotinamido)phenyl)porphyrinato)iron(III), (meso -a,a,a,a-Tetrakis( o -(N-methylnicotinamido)phenyl)porphyrinato)iron(III), and (meso-Tetrakis( 1-methylpyridinium-4-yl)porphyrinato)iron(111)k2 Gordon M. Miskelly, Wayne S . Webley, Charles R. Clark, and David A. Buckingham* Received December 9, 1987 The synthesis, or improved synthesis, of the parent porphyrin salts meso-a,a,a,a-tetrakis(o-(N-methylnicotinamido)phenyl)porphyrin triflate ( [a4-(TMNP)Hz](CF3S0J4), meso-a,p,a,~-tetrakis(o-(N-methylnicotinamido)phenyl)porphyrin perchlorate ([apap-(TMNP)H,] (CIO,),), and meso-tetrakis( 1-methylpyridinium-4-y1)porphyrintriflate ( [(TMPyP)Hz](CF3S03)4)are de[FeBr(apap-TMNP)Br4, [FeBr(TMPyP)]scribed, as is the insertion of iron to form [Fe(03SCF3)(a4-TMNP)](CF3S03)4, (CdBr4)2,and other salts. In aqueous solution at 25 OC the [Fe(P)(OH,)2]5+and [Fe(P)(OHz)(OH]4+ cations display the following pKalm,pKaZmvalues: 6.1, 10.28 (P = aPaP-TMNP, I = 0.1 mol dm-'); 5.8, 10.0 (P = a,-TMNP, I = 0.01 mol dm-,); 5.8, 11.7 ( P = TMPyP, I = 0.1 mol dm-,). Under the same conditions the [Fe(P)(OH)(OH2)]4+cations dimerize to form p-oxo-bridged species IFe(P))z08+with Kd values of (1.8 f 0.7) X lo7 dm3 mol-I (P = a,-TMNP), (7.35 f 0.1) X lo3 dm3 mol-' (P = TMPyP), and less than 1.2 X lo3 dm3 mol-' (P = a@@-TMNP, dimer not observed). Rate data for dimerization in the P = a,-TMNP, TMPyP systems have been obtained over the pH ranges 4.85-7.69 and 5.73-8.52, respectively, and have been interpreted in terms of dimerization of [Fe(P)(OH,),]S+ and [Fe(P)(OH)(OHz)]4+ units (k12= 1.2 X lo4 mol-' dm3 s-' (0.01 mol dm-) NaNO,) and 1.06 X IO3 mol-' dm3 s-I (0.1 mol dm-) NaNO,), respectively, at 25 "C). Hydrolysis of these dimers has been studied at p H 2.05-5.3 (P = a,-TMNP) and pH 1.38-10.48 and 0.01-0.50 mol dm-3 NaOH (P = TMPyP) and has been interpreted in terms of hydrolysis of [(Fe(P))z(OH2)]'o+(kh,!Kald = 26.6 mol-' dm3 s-' (P = TMPyP)) and [(Fe(P))z(OH)]g+(kh2 2 40 s-' (P = a,-TMNP), 0.355 s-I ( P = TMPyP)) units under acidic conditions, and first order in [OH-] hydrolysis of [(Fe(TMPyP))zO]8+ in alkali (kh3= 2.9 X lo3 mol-' dm3 s-', 0.1 mol dm-' N a N 0 3 , 25 "C). Comparisons where appropriate are made with data recently reported by Tondreau and Wilkins. stricted toward further coordination and F ~ ( C ~ ~ - T M Nhas P )one ~+ face restricted, while F ~ ( T M P Y P ) h~a+s both faces open toward coordination by a fifth and sixth ligand. The effect such restrictions have o n t h e acidity of coordinated water molecules and o n t h e rate and ability t o dimerize are important issues in relation t o t h e behavior of biologically important Fe(II1) porphyrins.5
Introduction The recent pHstopped-flow studies by El-Awady, Wilkins, and Wilkins3 and Tondreau and Wilkins4 on t h e dimerization a n d hydrolysis reactions of the water-soluble iron(II1) porphyrins represented by (1) and (2)2 prompts us t o report our similar studies on t h e t w o cationic systems [Fe(apcup-TMNP)(OH2),I5+ a n d [Fe(cu4-TMNP)(OH2),l5+. 2 [ Fe(TSPP) (OH,),]
+
+
+
ki
3-
Z k-i
[(TSPP)Fe-0-Fe(TSPP)l8-
+ 2H+ + H 2 0 (1)
kl
2[ F ~ ( T M P Y P ) ( O H ~s+) ~e ] k-1
[(TMPyP)Fe-O-Fe(TMPyP)I8+
+
+ 2H+ + H 2 0 (2)
We also report o u r own observations on t h e F ~ ( T M P Y P ) ~ + system since, although largely confirming those by Tondreau a n d Wilkins? they were obtained under somewhat different electrolyte conditions and cover a wider pH range. The three metalloporphyrins 1-111 form a graded series in which Fe(afia/3-TMNP)5+has both sides of t h e porphyrin plane re(1) Abstracted in part from: Webley, W. S. Ph.D. Thesis University of Otago, May 1984. Miskelly, G.M. Ph.D. Thesis, University of Otago, Oct 1986. The work described here formed the basis of a Conference Plenary Lecture, Royal Australian Chemical Institute (COMO-12), Hobart, Australia, Jan 1984. (2) Abbreviations used for coordinated porphyrin dianions: TMPyP = tetrakis(1-methylpyridinium-4-y1)porphyrinato;TSPP = tetrakis(4su1fonatophenyl)porphyrinato;TMNP = tetrakis(0-(N-methylnicotinamido)phenyl)porphyrinato; a4 and aPaP designate atropisomers; TTMPP = tetrakis(3,4,5-trimethylphenyl)porphyrinato; TCPP = tetrakis(4-carboxylatopheny1)porphyrinato;TMPP = tetrakis(4-methylpheny1)porphyrinato; TMIPP = meso-tetrakis(2,4,6-trimethoxypheny1)porphyrinato; TM,PP = meso-tetramesitylporphyrinato; TSDMPP = tetrakis(2,6-dimethyI-3-sulfonatophenyl)porphyrinato; DCMPIX = 2,4-dicysteinyl-meso-porphyrinatoIX; PPIX = protoporphyrinato IX; DPIX = deuteroporphyrinato IX; DSDPIX = 2,4disulfonatoprotoporphyrinato IX; TEnPPIX represents the tetraethylenediamine derivative of the protoporphyrinato IX dianion (see: Kolski, G. P.; Plane, R. A. J . Am. Chem. Soc. 1972, 94, 3740); EDTA represents ethylenediaminetetraacetate. (3) El-Awady, A. D.; Wilkins, P. C.; Wilkins, R. G. Inorg. Chem. 1985, 24, 2053. (4) Tondreau, G. A,; Wilkins, R. G. Inorg. Chem. 1986, 25, 2745.
0020-1669/88/1327-3773$01.50/0
Fe( T M Py P?' (111) 5t
F e ( a Paa-TMN P )
(I)
Experimental Section Preparations. Reagents were LR or AR grade where available, trifluoromethanesulfonic acid (Fluorad FC24, 3M Co.), meso-tetrakis(4pyridy1)porphyrin (TPyPH2, Strem Chemicals), and trimethyl phosphate (Koch-Light), with the last being dried over 4A sieves. Methyl trifluoromethanesulfonatewas prepared by adding trifluoromethanesulfonic acid (100 cm', 1.13 mol) cautiously from a dropping funnel protected by a guard tube containing silica gel to freshly distilled (76 OC, 15 mmHg) dimethyl sulfate (100 cm', 1.06 mol). The methyl trifluoromethanesulfonate was then fractionally distilled (2 X 15 cm fractionation column) at 97 " C under anhydrous conditions to yield 107 g (97%) of a clear liquid, which was stored in a stoppered flask protected from light. (Caution! methyl trifluoromethanesulfonate is potentially extremely hazardous; inhalation of vapor and contact with skin should be avoided:) Thin-layer chromatography on aluminum-backed Kieselgel 60F254 (Merck Art 5562) was used to monitor porphyrin syntheses and to check product purity. Methylation of tetrapyridylporphine and insertion of iron has previously been reported by Fleischer and Hambright' and by Harris and Toppen: and an IR assessment of the methylated complex has been given (5) Perutz, M. F.; Fermi, G.; Luisi, B.; Shaanan, B.; Liddington, R. C. Acc. Chem. Res. 1987, 20, 309. (6) A fatal accident involving a small quantity of the related ester, methyl fluorosulfonate, is reported in: Van der Ham, D. M. W.; Van der Moor, D. Chem. Eng. News 1976, 54(36), 5. (7) Fleischer, E. B.; Hambright, F. Inorg. Chem. 1970, 9, 1759. (8) Harris, F. L.; Toppen, D. L. Inorg. Chem. 1978, 17, 71.
0 1988 American Chemical Society
3774 Inorganic Chemistry, Vol. 27, No. 21, 1988 by Forshey and K ~ w a n a . ~The following synthetic method has been found to be more satisfactory and avoids the hazards of potentially explosive perchlorate salts. meso -Tetrakis(1-methylpyridinium-4-yl)prphyrin Trifluoromethanesulfonate, [(TMPyP)H2](CF3S0,),. (TPyP)H2 (1 .O g) was suspended in dry trimethyl phosphate (15 cm3) containing 2.6-lutidine (0.25 8). Methyl trifluoromethanesulfonate (2.0 g) was added and the mixture stirred in a stoppered flask for 5 min. Further 2,6-lutidine (0.15 g) was then added to the now green-brown solution, and this mixture was stirred for 1.5 h before being poured into water (300 cm’). The aqueous mixture was cooled at 0 OC for 30 min, and the resulting purple crystalline solid was removed and washed with water (2 X 100 cm’), 2-propanol (2 X 30 cm’), and ether (50 cm3) and air-dried (yield 1.90 g, 93%). This product was recrystallized from hot methanol (450 cm3) by adding hot (60 “C) 2-propanol (600 cm’) and cooling in ice. The crystalline purple product (1.74 g, 85%) was filtered off, washed with 2-propanol and ether, and dried in air. Anal. Calcd for C48H38N8012F12S4: C, 45.2; H, 3.00; N , 8.79; F, 17.9; S, 10.0. Found C, 45.4; H , 3.35; N , 8.85; F, 17.9; S , 10.2. Electronic spectrum ( e , lo4 mol-’ dm’ cm-’ (A, nm); MeOH): 25 (424), 1.8 (514), 0.63 (590), 0.14 (647). ‘ H N M R (90 MHz, Me2SO-d6): 6 9.8 (d, 8 H, pyridyl CH), 9.47 (s, 8 H, pyrrole CH), 9.30 (d, 8 H, pyridyl CH), 4.93 (s, 12 H, methyl CH), -2.90 (s, 2 H, pyrrole NH). TLC: RJ 0.17 (H,O:HOAc:py:MeOH, 2:2: 1:1). meso -a,a,a,a-Tetrakis( o -(N-metbylnicotinamido)phenyl)porphyrin Trifluoromethanesulfonate, [a4-(TMNP)H2](CF3S03)4. To a stirred (a4suspension of a,a,a,a-tetrakis(o-nicotinamidophenyl)porphyrin’o (TNP)H2, 2.0 g) in dry trimethyl phosphate (20 cm’) was added methyl trifluoromethanesulfonate (2.5 g). After 30 min the green solution was poured into water (400 cm’) and the resultant purple precipitate filtered off, washed with water (3 X 50 ern'), and dried under vacuum. The porphyrin was crystallized from methanol (300 cm’) following addition of 2-propanol and slow reduction of the solution volume (rotary evaporator, 25 “C) to 100 cm’. The crystalline product (2.4 g, 75%) was filtered off, washed with 2-propano1, and air-dried. Anal. Calcd for C76H~8N12016F12S4:C, 52.1; H , 3.3; N , 9.6; F, 13.0; S,7.3. Found: C, 52.4; H, 3.4; N , 9.4; F, 13.1; S, 7.6. ‘H N M R (90 MHz, Me2SO-d6): d 10.97 (s, 4 H, amide N H ) , 8.98 (s, 8 H , pyrrole CH), 8.85-7.53 (m, 32 H, phenyl and pyridyl CH), 4.13 (s, 12 H , CH3), -2.88 (s, 2 H, pyrrole NH). TLC: R, 0.20 (H20:HOAc:py:MeOH, 2:2:1:1). meso -a&,@-Tetrakis(o - (N-methylnicotinamido)phenyl)porphyrin Perchlorate, [a@a&(TMNP)H2](C104)4.To a stirred solution of the mixed atropisomers of tetrakis(o-nicotinamidophenyl)porphyrinIo ((TNP)H2, 2.86 g) in dry trimethyl phosphate (30 cm’) was added methyl trifluoromethanesulfonate (3 cm’) and 2,6-lutidine (2 cm’). After 30 min further methyl trifluoromethanesulfonate (1 cm’) and 2,6-lutidine (0.5 cm’) were added and the solution was stirred for another 45 min before the reaction was quenched by addition of methanol (20 cm’). The resultant solution was slowly poured into rapidly stirred ether (500 cm’) and the precipitated product isolated by filtration on Celite. It was washed from the filter cake with methanol and reprecipitated by addition of 2-propanol and ether. The isolated solid was dissolved in HC1 (800 cm’, 0.75 mol dm-’) and the filtered green solution degassed (nitrogen bubbling) and irradiated with visible light” (500-W tungsten lamp, 200-W Hg lamp equipped with soda-glass filter) in a flask fitted with a cold-finger condenser, stirrer, and nitrogen inlet. After 10 h TLC (H,O:HOAc:py:MeOH, 2:2:1:1) showed that >90% of the tetrakis(onicotinamidopheny1)prphyrin was in the a,&~,p conformation. The solution was stirred and NaC104 added until a green precipitate began to form. The suspension was made alkaline (“,OH), and the precipitated porphyrin was filtered off, washed thoroughly with water, then with 2-propano1, and finally with ether. The resulting hygroscopic purple solid was taken up in water (1 dm’) containing “,OH sp gr 0.91, 10 cm3) and the solution sorbed onto Sephadex SP-C25 ion-exchange resin (30 X 7 cm). The column was washed with N H 4 0 H (2 mol dm-’) and the porphyrin removed by gradient elution (2 mol dm-3 ”,OH and NaCl (0.3 mol dm-’) containing 2 mol dm-’ “,OH). The product was isolated following adjustment to pH 7 (HCI04) and addition of excess NaCIO,. Recrystallization was effected by slow evaporation (rotary evaporator) of a methanol-acetone-2-propanol solution. Anal. Calcd for C72HS8N12020:C, 55.7; H, 3.8; N, 10.8. Found: C, 53.6; H, 3.6; N , 10.8. Electronic spectrum ( e , lo4 mol-’ dm’ cm-’ (A, nm); MeOH): 32.6 (422), 1.63 (517), 0.49 (551), 0.52 (592), 0.18 (647). TLC: RJ0.40 (H,O:HOAc:py:MeOH, 2:2: 1: 1). Bromo(meso-tetrakis(I-methylpyridinium-4yl)porphyrinato)iro(III) Salts. Method a. To [(TMPyP)H2](CF3S03),(200 mg) in pyridine (5 ~~~~~
(9) Forshey, P. A.; Kuwana, T. Inorg. Chem. 1983, 22, 699. (IO) Buckingham, D. A,; Gunter, M. J.; Mander, L. N. J. Am. Chem. SOC. 1978,100,2~9.
(11) Freitag, R. A,; Whitten, D. G. J . Phys. Chem. 1983, 87, 3918.
Miskelly et al. cm’) and acetic acid (10 cm’) was added aqueous FeS04 (0.3 cm’, saturated solution). The resulting solution was maintained at 90 OC under nitrogen for 30 min. The solvent was removed (rotary evaporator, 90 OC, 4 h) and the residue redissolved in HBr (0.1 mol dm-’, 15 cm’), and this solution filtered. Adding aqueous LiBr (saturated solution) to the filtrate at 60 OC and subsequent slow cooling resulted in the formation of purple-black crystals, which were removed and washed with 2propanol and air-dried. The purple crystalline solid (137 mg, 89%) was recovered after recrystallization from hot methanol-2-propanol solution. 6.6H20: C, 42.0; H, 3.97; Anal. Calcd for [Fe(TMPyP)](CF3S0,)o,4Br3 N , 8.83; Br, 28.9; S, 1.01. Found: C, 40.8; H, 3.69; N , 8.71; Br, 30.7; S, 0.58. Electronic spectrum ( e , 10’ mol-’ dm’ cm-’ (A, nm); 0.1 mol dm-’ “0’): 95.3 (400), 8.78 (480), 10.3 (520), 7.99 (536), 3.85 (560), ~ 3.15 (580), 2.92 (595), 3.41 (630) (cf. em = los mol-’ dm’ ~ m - ’ ) .TLC: RJ 0.12 (H,O:HOAc:py:MeOH, 2:2:1:1). Method b. Cd2+-Catalyzed Iron Insertion. To [ (TMPyP)H2],(CF3S03)4(103 mg) in methanol-water ( l : l , 20 cm’) containing pyridine (0.2 cm’) was added Cd(N03)2.4H20 (86 mg). The solution changed from red-purple to green (3 min) and was deoxygenated (15 min of Ar bubbling), following which FeCl2.4Hz0 (131 mg) was added. Stirring for 3 h under Ar resulted in a red solution. This was acidified (red to yellow color change) and diluted to 75 cm’ with water and the porphyrin precipitated by addition of LiBr (1 cm’, saturated solution). The precipitate was collected on a Whatman GFC filter and washed with 2-propanol. The porphyrin in boiling water (50 cm’) was crystallized by addition of LiBr (1.5 cm’, saturated solution) followed by slow cooling to 0 OC. Purple crystals (59 mg) were filtered off, washed with 2propanol and ether, and dried in air. Anal. Calcd for [FeBr(TMPyP)](CdBr,),: C, 32.5; H , 2.28; N , 6.68; Cd, 13.4; Br, 42.8. Found: C, 31.9; H, 2.60; N , 6.35; Cd, 12.8; Br, 42.7. Analysis also indicated the presence of some S (0.70%). This could not be explained by the presence of CF3S03-as the appropriate IR bands were absent. The cadmium was removed as follows. To [FeBr(TMPyP)](CdBr,), (250 mg) in water (150 cm’) was added NaOH solution (2 cm’, 5 mol dm-’). The resulting white precipitate was removed on a Whatman GFC filter paper, and aqueous LiBr (4 cm’, saturated solution) was added to the filtrate. This was then acidified with HBr (48%), reduced in volume to 25 cm’, and cooled on ice. The resulting purple crystals (125 mg) were collected, washed with 2-propano1, and air-dried. Anal. Found: Cd, 4 X low5 mol dm-3 at pH 3.0 is jumped to pH >5.6, the equilibrium position now favors the M-oxodimer. Figure 3 shows such a second-order approach to equilibrium at pH 7.69. This process was investigated over the pH range 5.52-7.69 and for [FeP], = (4.47-6.55) X lo-' mol dm-3. Second-order rate constants ( k f )were obtained from linear plots of ([MI, - [MI,)-' vs time, and these are given in Table 111 (supplementary material). The reaction of the TMPyP dimer [{Fe(TMPyP))20]8' was similarly studied with first-order data being obtained over the complete pH range 1-13. Those data from pH 1 to 6, and those above pH 10.5, were independent of dimer concentration (7.5 X 3.57 X mol dm-3) and therefore represent kh. Such data are given in Table IV (supplementary material). For the pH range 6-10.5 the data (Table V, supplementary material) show a concentration dependence and therefore contain both kh and k p Under such conditions the more generally useful expression (9) l6,I7 was = kobsd2= 8kfkh[Fe(TMPyP)],
T - ~
+ kh2
(16) Bernasconi, C.F. Relaxation Kinelics; Academic: London,
(9)
1976.
Discussion Hydrolysis and Dimerization Equilibria. The equilibrium, kinetic, and electrochemical data39support the existence of three monomeric, and up to three dimeric, species for the iron(II1) porphyrins with P = a4-TMNP and TMPyP in aqueous solution. Scheme I depicts these. Only three monomeric ions exist for the P = apap-TMNP system. The protonated dimers [(Fe(P))20H]w and [(Fe(P)J20H2]'o+were not directly observed although significant quantities of [(Fe(TMPyP))20H]9' are required by the kinetic results in the pH range 4-6. Our equilibrium data are compared with literature data in Table VI. The acidities of [Fe(P)(OH2)2]5+,P = aPaP-TMNP (pK,," = 6.09), P = a4-TMNP (5.79), and P = TMPyP (5.79), are similar with these highly positively charged metalloporphyrins being more acidic than those of lower positive, or negative, charge. The pK,,"' values for P = aPaP-TMNP (10.28) and (r4-TMNP (10.0) show that their aqua-hydroxo ions are more acidic than [Fe(TMPyP)(OH2)(OH)]4+(pK,,"' = 11.71), suggesting that coordination to the second water molecule in the latter is appreciably weaker. Few certain deprotonations of a second coordinated water have been observed with monomeric iron(II1) systems (cf. Table VI). This is usually because dimerization is both rapid and reasonably complete at high pH. Also, it is suggested from this study that the spectral changes associated with a second deprotonation may not be overly large (Figure 1). Such weak acidity of a second coordinated water supports the generally accepted view of a distorted coordination environment with the Fe(II1) atom being held somewhat out of the porphyrin plane toward the other electronegative ligand (by up to 50 pm for OH-, Cl-, OMe-).1s319 It is unlikely that the water molecules in [Fe(P)(OH2)2]5+are replaced by electrolyte anions since spectra obtained with the same concentrations of NaC1, NaBr, NaN03, and Na03SCF3were indistinguishable, whereas significant changes did occur with F, N3-, CN-, and SCN-. Also, the structure of [Fe(TMPyP)(OH2),]Br5 in the solid state clearly contains two trans-coordinated water molecules.20 Whether or not the monohydroxo system has its sixth coordination site occupied by water is less ~ e r t a i nbut ,~ we prefer to include it since the corresponding cobalt(II1) complex [CO(TMP~P)(OH,),]~+ has similar pK,,"' and pK,,"' values (5.5 (17) It can be shownz1that for an error of 40 (a4-TMNP), 8.0 (TSPP),j 0.36 s-l (TMPyP)). Supplementary Material Available: Observed rate constants for the hydrolysis and relaxation of [IFe(a4-TMNP)}20]8+at pH 2.05-5.32 (Table 11), for the formation of [(Fe(a4-TMNP)}20]s+at pH 5.64-7.69 (Table HI), for the hydrolysis of [(Fe(TMPyP)J20]*'at pH 1.38-10.48 and 0.01-0.50 mol dm-3 NaOH (Table IV), and for the relaxation of [(Fe(TMPyP)}20]8' at pH 5.75-9.62 (Table V) (6 pages). Ordering information is given on any current masthead page.
(36) Fielding, L.; Eaton, G. R.; Eaton, S. S. Inorg. Chem. 1985, 24, 2309. (37) Buckingham, D. A. In Biological Aspects of Inorganic Chemistry; Wiley-Interscience: New York, 1977; Chapter 5 . (38) Lind, M. D.; Hoard, J. L. Inorg. Chem. 1964, 3, 34. (39) Buckingham, D. A,; Clark, C. R.; Miskelly, G. M.; Webley, W. S., submitted for publication in J . Electroanal. Chem. Interfacial Electrochem.
